“What would we be like without the national parks?” Ken Burns calls that the “It’s a Wonderful Life” question, the one raised in the classic Christmas movie. What if Americans hadn’t stepped up to save their special places?

“If there were no national parks, the Grand Canyon would be lined with mansions inaccessible to us mere mortals,” Mr. Burns says. “If there were no national parks, Zion and Yosemite – two of the most beautiful places on earth – would be gated communities. If there were no national parks, Yellowstone would be an amusement park. If there were no national parks, the Everglades would be drained and filled with subdivisions and tract housing.”

As Americans moved westward, many saw resources to be exploited, says the documentary filmmaker, whose achievements include “The Civil War,” “Baseball,” and “Jazz.” “We look at a river and think ‘dam.’ We look at a beautiful stand of trees and think ‘board feet.’ We look at a canyon and think what minerals can be extracted from it.”

But another strain also wound through the American mind, he says, one that saw in nature “cathedrals of God’s handiwork” and surmised that “we could do no better than to return to nature for some sense of touching something larger than ourselves.”

Those impulses finally resulted in the formation of the National Park Service in 1916. But that event, Burns points out in an interview with the Monitor at the WGBH studios in Boston, isn’t covered until midway into the third of the six, two-hour episodes of his latest project, “The National Parks: America’s Best Idea,” which began airing on PBS Sept. 27.

While some might argue that Jefferson’s “all men are created equal” is America’s best idea, the nation’s founding principles lie beneath the idea of creating parks. “Only a democracy could have thought that land could have been set aside, not for the rich and nobility, but for everybody for all time,” Burns says.

The effort began by saving the obvious: the greatest waterfall (Yosemite), the greatest canyon (the Grand Canyon), the greatest geysers (Yellowstone).

But the idea has expanded to include sites of historical and cultural importance, from hallowed Civil War battlefields to places of national shame (Indian massacres) or tragedy (the Oklahoma City National Memorial & Museum, honoring the victims of the 1995 terrorist bombing).

While “National Parks” dazzles with spectacular cinematography shot over six years, in typical Burns fashion it’s also the intimate story of individuals. You meet the “famous white guys,” such as Teddy Roosevelt and John Muir, he says. But it’s also a “bottom up” story “that’s black and brown and red and yellow and female and unknown.” That diversity wasn’t the result of being politically correct, he says. Rather, it naturally occurred “by lifting up the rock at any given park and looking at its story.”

The concern that Americans may be “loving their parks to death” by crowding into them has been tempered by flat attendance figures (275 million visitors in 2008) in recent years, though Yellowstone reported record crowds this summer.

Burns argues that encouraging more Americans to visit their national parks is the best way to protect them. As a result of the PBS series, “I hope that every superintendent of every national park ... [will] be angry with us because they have so many people, and they don’t know what to do with them,” he says.

Americans are “co-owners of some of the most spectacular scenery on earth,” Burns says. “And all you’ve got to do is go out and visit your property now and then, and make sure it’s being taken care of. And put it in your will for the next generation.”